Yeah, this sounds like textbook "Treatment Emergent CSA". It's supposed to eventually go away in OSA cases after continued use with the CPAP, so if it dropped your amount of obstructives to be lower than the amount of centrals, that is proof positive that it is necessary.
On my original sleep study with no PAP titration, I had 6 obstructive apneas, 19 central apneas, 2 mixed apneas and 31 hypopneas. Because of the inability to distinguish a "central hypopnea", they label them as obstructive and boom, I was labeled OSA. In my case though, the sleep doc noted that the amount of central apneas was abnormal and my obstructive apneas disappeared with my BiPAP ASV, while centrals lowered but were still persistent (AHI of 3.5 avg). Therefore, she deemed it "more likely than not TBI related CSA".
I believe this is the KEY for 50%. A word of caution on the word "recommended" though because I feel like it isn't strong enough. "Required" or the other words mentioned would be more definitive and not leave much wiggle room.
However....PAP treatment may still end up being listed as an option because there are "mouth guards" that could be used for OSA. I have an extremely sensitive gag reflex and couldn't even stand a mouth guard in sports, so that was a no go for me. Not sure if a mouth guard would be effective for your situation, but if it isn't make sure the doctor notes it or try to find literature that suggests why it was not chosen for treatment.
I do not believe the mouth guard counts for the 50%, but I could very well be wrong. As far as the CPAP, I read that the VA classifies other positive airway pressure devices (AutoPAP, BiPAP, etc.) as equivalent to a CPAP.
Despite my VSOs hesitation due to my recent 100% P&T, I'll likely be heading to the BVA on my denial because if this apnea were to make me kick the bucket, I want the service connection.
I sincerely hope you don't have to do the same for your claim!